Oxides of nitrogen (NOX) that are emitted by an emissions source, such as those formed as a result of combustion, are included among the main causes of the “acid rain” problem, the photochemical smog problem and the resulting damage to the environment. These harmful substances should therefore be eliminated to the greatest extent possible from the gases emitted by an emissions source, such as the exhaust from a combustion process, prior to their discharge into the atmosphere.
One source of nitrogen oxides, in the form of NO2 and mainly NO, are those formed by the combustion of coal, oil, gas, gasoline, diesel fuel or other fossil fuels. Combustion of fossil fuels occurs, for example, in a stationary device such as furnace, which is a device for the production or application of heat. A furnace may be used in connection with a boiler such as in a steam generator that drives a steam turbine in an electrical generating plant, in connection with an industrial operation such as in a smelter or chemical reactor, or in connection with supplying heat for human consumption.
Fossil fuels are also combusted in a mobile device, including a device that supplies mechanical power such as an internal combustion engine in a vehicle for transportation or recreation, or in a piece of equipment for construction, maintenance or industrial operations; or in a gas turbine, which is a turbine driven by a compressed, combusted fluid (such as air), such as in the engine of a jet aircraft. Gas-emitting devices such as an internal combustion engine or a gas turbine are also found in stationary applications, however. The exhaust gas emitted by devices such as those described above is a multi-component mixture of gases containing nitrogen oxides. Nitrogen oxides are also emitted by plants for the incineration of industrial or municipal waste. In addition, carbon monoxide and hydrocarbons are also emitted by these sources.
A problem exists with respect to the need for control of the injection of a reducing agent into a gas mixture containing nitrogen oxides. There is a desire to effect the reduction of as large a quantity of the nitrogen oxides present in the gas mixture as possible. For this purpose, what amounts to a stoichiometric excess of reducing agent, in terms of the quantity of nitrogen oxides present, is often injected into the gas mixture and thus into the nitrogen oxides. An excess of reducing agent is employed not so much by design but primarily because of the unavailability of information related to the compositional content of the gas mixture sufficient to accurately calculate the stoichiometric equivalent of reducing agent needed. The compositional content of a gas mixture containing nitrogen oxides often varies in an extremely unpredictable manner as it moves through a conduit from its emission source to the point of its ultimate destination, such as a point of discharge into the atmosphere. As a result, because of the desire to obtain reduction of a large percentage of the nitrogen oxides, an amount of reducing agent is injected that later proves to be an excess. Whether this results from calculations based on inaccurate or incomplete information, a strategy of employing an excess to be certain that too little is not employed, or incomplete reaction of whatever the amount, the same undesired consequence is experienced—unreacted reducing agent is discharged to the atmosphere and becomes a pollutant itself. When ammonia is the reducing agent, this is known as ammonia slip. In a gas mixture that is unscrubbed, or otherwise contains sulfur oxides, unreacted ammonia is also capable of reacting with the sulfur oxides to yield corrosive, sticky deposits of ammonium sulfate and/or ammonium hydrogen sulfate that foul the mechanism of the conduit.
There is a need then for a method and apparatus for the reduction of a nitrogen oxide that provides control of the reaction of reduction, and in particular control of the injection of a reducing agent into the gas mixture containing the nitrogen oxide. In particular, there is a need for a method and apparatus that enables the calculation of the amount of reducing agent to be injected in relation to information about the compositional content of the gas mixture.
This invention addresses those needs by providing a method and apparatus in which analysis of the gas mixture is performed to furnish information related to the compositional content thereof. In certain embodiments, the analysis is furnished by a gas analyzer that may be placed within a conduit through which the gas mixture is transported in positions that create an opportunity to develop useful information about the gas mixture, and especially information related to the nitrogen oxide content thereof. In certain other embodiments, a gas analyzer is employed for this purpose that outputs a signal related to the content within the gas mixture of an individual component gas therein and/or the collective content of a sub-group of gases therein. In certain other embodiments, the information is inputted into a decision making routine and/or a map, and may be used to calculate a desired amount of reducing agent to be injected into the gas mixture, and thus into the nitrogen oxides to be reduced. Other embodiments of the invention are as more particularly described below, or are as would be apparent to the artisan in view of the description below.